IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services which are considered in more detail below.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over 3G mobile communication networks (3GPP TS 23.228 and TS 24.229 Release 5 and Release 6). IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and network servers). The Session Description Protocol (SDP), or other protocol, carried by SIP signalling, is used to describe and negotiate the media components of the session. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), Hyper Text Transfer Protocol (HTTP). IMS requires an access network which might be for example a 2G/3G General Packet Radio Service (GPRS)/Packet Switched (PS) network, but which might be some other access network such as fixed broadband or WiFi network. FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a GPRS/PS access network.
The IMS services which a SIP enabled user terminal can access will depend upon the capabilities of the terminal. For example, a terminal will only be able to make use of a picture sharing service if the terminal has a camera and appropriate photo functionality. SIP, as defined in IETF RFC 3261, provides a so-called SIP OPTIONS mechanism which allows the SIP client of a terminal to determine the capabilities (e.g. supported applications, codecs, etc) of some other terminal. The mechanism requires the sending of a SIP OPTIONS method from a first to a second terminal, and the sending of a response, 200OK, in the reverse direction.
It is noted that SIP is applicable to services other than those facilitated by IMS. It is also noted that terminal capability exchange is a requirement of multimedia setup and control protocols other than SIP. For example, the International Telecommunication Union Telecommunications Sector (ITU-T) has defined the standard H.324 which provides a mechanism for capability exchange.
It is anticipated that in the very near future SIP functionality will be implemented in mobile cellular terminals. In order to allow subscribers to maximise their use of the available IMS services, this SIP functionality will, sooner or later, include the SIP OPTIONS mechanism. However, the sending of a SIP OPTIONS message and a response between terminals is likely to significantly increase the consumed signalling resources.
In order to reduce the SIP signalling requirements, it has been proposed (3GPP TR 23.899 version 0.5.0) to store or “cache” the result of an initial capability query facilitated by SIP OPTIONS, at mobile terminals. The initial capability query may be carried out the first time that a given terminal tries to set up a SIP call to another terminal. For any subsequent call attempts, the calling and called terminals extract information from their respective caches to determine peer capabilities. it is noted that similar proposals have been presented for H.324.